1. Field of the Invention
The present invention relates to a radiation detector and sample analyzer.
2. Description of Related Art
A radiation detector is an instrument for detecting X-rays, gamma rays, or other radiations. Radiation detectors include known X-ray detectors used to detect X-rays.
Known types of X-ray detectors include energy-dispersive X-ray spectrometers (EDS) and wavelength-dispersive X-ray spectrometers (WDS).
An energy-dispersive X-ray spectrometer detects X-rays emanating, for example, from a sample by means of a semiconductor detector, converts the detected X-rays into an electrical signal, and performs spectroscopic analysis.
For example, in an X-ray fluorescent analyzer (XRF) fitted with an energy-dispersive X-ray spectrometer disclosed in JP-A-10-318946, the output signal from a semiconductor detector is amplified to a given level by a preamplifier and then wave shaped into pulses having heights corresponding to energy values of characteristic X-rays by a wave-shaping circuit. The pulses are sent as a detected X-ray signal to a multichannel analyzer. The analyzer obtains a pulse-height spectrum (also known as an energy spectrum or an X-ray spectrum) by calculating pulse heights indicated by the detected X-ray signal and discriminating the pulse heights. In this energy spectrum, peaks intrinsic to individual elements appear at positions corresponding to the energy values of characteristic X-rays released from the elements contained in the sample.
This energy-dispersive X-ray spectrometer has the problem that sum peaks appear in the obtained X-ray spectrum.
In an energy-dispersive X-ray spectrometer, if a plurality of X-rays impinges on a semiconductor detector within a short interval of time, adjacent pulses interfere with each other, producing an apparently one pulse, so-called pileup. That is, these pulses cannot be recognized as individual pulses. As a result, a peak appears at the sum position of the energies of plural X-rays, apart from spectral peaks from the sample. This peak is a sum peak.
FIG. 21 is a graph showing one example of an X-ray spectrum obtained by measuring a sample containing Mn by an energy-dispersive X-ray spectrometer.
In the X-ray spectrum shown in FIG. 21, a peak appears near the Mn—Kα line (5.9 keV). In addition, a sum peak appears near 11.8 keV that is the sum of the energies of two Mn—Kα lines. Such a sum peak causes incorrect qualitative and quantitative analyses.
In order to suppress sum peaks, it is necessary to enhance the accuracy at which pileup is identified. For example, JP-A-2009-229127 discloses a pulse processor that is configured including a wave-shaping circuit for producing a pulsed signal having pulse heights corresponding to energy levels indicated by an event signal having energy information, a pulse height measuring circuit for measuring the pulse heights of the pulsed signal, a plurality of event detection circuits for producing a first timing signal in response to the event signal, an OR circuit for outputting a second timing signal by ORing the outputs from the event detection circuits, and a pileup detection circuit provided behind the OR circuit.
In the pulse processor of JP-A-2009-229127, if the intensity of the radiation increases and the frequency at which the first timing signal is output using the plural event detection circuits increases, the second timing signal is produced at shorter intervals of time. In this case, pulsed signals delivered from the wave-shaping circuit have more overlap with one another, resulting in pulse pileup. When the time interval between the temporally adjacent portions of the second timing signal is shorter than the time in which each pulse height is measured and processed, a pileup detection circuit determines that pileup has occurred and outputs a signal for stopping the measurement of pulse heights. Consequently, when it is determined that pileup has occurred, the resulting pulse heights are not reflected in the X-ray spectrum. This suppresses generation of sum peaks.
However, even where the event detection circuit has a time constant shorter than that of the wave-shaping circuit, if plural X-rays enter within a short interval of time, pileup may occur in this event detection circuit. If so, in the pulse processor of JP-A-2009-229127, the first timing signal is not output precisely. In particular, if pileup occurs in the event detection circuit, only one cycle of second timing signal is output in response to two events. Therefore, the pileup detection circuit cannot precisely identify pileup. Sum peaks will appear in the X-ray spectrum.